1. Field of the Invention
This invention relates to a process for the production of an orientation layer on a plane surface of a plate. The invention also pertains to an arrangement for implementing this process as well as to a liquid crystal substrate plate.
2. Discussion of Background
A process for producing an orientation layer on a plane surface of a plate, wherein the plane surface is exposed to a diverging particle stream impinging on the surface at an angle of incidence other than 0.degree., is known from U.S. Pat. No. 3,834,729.
Such a process is especially suitable for the production of orientation layers on the substrate surfaces facing the liquid crystal substance in liquid crystal displays. For example, through the oblique evaporation of SiO on the surfaces, structures are produced which approximate arrangements of slanting, column-like elements (see L. A. Goodman et al.: Topography of Obliquely Evaporated Silicon Oxide Films and its Effect on Liquid Crystal Orientation, IEEE Transactions on Electron Devices, Vol. ED-24, No. 7, 1977, pp. 795-804). These structures orient the optic axis of the liquid crystal on the boundary layer adjacent to the substrate surface parallel to a preferred direction and thereby, on the basis of the coherence inside the liquid crystal, essentially control the configuration in the whole display cell.
In the manufacturing process known from U.S. Pat. No. 3,834,792, the substrate plates to be coated are arranged in a vacuum apparatus in pairs above an evaporation source, which is essentially a point source, so that the particle stream emanating from the source impinges upon the surfaces of the plates with an angle of incidence of about 80.degree. measured with respect to the plate normal. The relative spatial position of the plates and the source does not change during the evaporation operation.
The preferred direction of the optic axis at every point on the surface, which is determined by the orientation layer, not only depends on the thickness of the orientation layer but is also especially sensitive to the incident direction of the particle stream emanating from the source. The pretilt angle between the preferred direction and the surface depends upon the angle of incidence of the particle stream (see: T. J. Scheffer and J. Nehring: Accurate determination of liquid-crystal tilt bias angles, J. Appl. Phys., Vol. 48, No. 5, 1977, pp. 1783-1792). The azimuthal orientation of the preferred direction, on the other hand, coincides with that of the incident particle stream, and because of the spatial divergence of the particle stream, a uniform alignment of the preferred direction over the surface of the entire plate can only be achieved by this means if the separation between the evaporation source and the surface is very large in comparison with the dimensions of the plate.
Such a uniformity is, at least regarding the azimuthal orientation (nonuniformity in the pretilt angle has a lesser effect) absolutely necessary in order to achieve satisfactory optical properties of the liquid crystal display.
For example, in order to produce an orientation layer having a tolerable azimuthal variation of less than 5.degree. in a large area matrix display with 192 points.times.280 points and an active area of about 14 cm.times.20 cm, a source-substrate separation of more than 230 cm would be required using the known process. This kind of separation can be realized only in large vacuum chambers with correspondingly increased installation costs, longer pumping times and low plate throughputs.
The known process and the usual evaporation distances of about about 30 cm which are attainable in the smaller vacuum chambers lead to a variation in the azimuthal orientation of about 30.degree. for plates with the dimensions given above. Variations on this order with the accompanying variations in twist angle of the liquid crystal layer, however, cause unacceptable variations in the color, operating voltage and response time in the finished display cell.
In another known process (N. Koshida: Large-area quasihomeotropic orientation of liquid crystal and its application to guest-host positive display, J. Appl. Phys., Vol. 52, No. 9, 1981 pp. 5534-5536) an extended source having a length of about 40 mm is used instead of the point source. It is clear, however, form geometrical considerations that only an infinitely extended source can lead to a really uniform azimuthal orientation of the preferred direction over the entire evaporated surface. A truncated linear extension of the source can only moderate the nonuniformities. In addition, extending the source increases the nonuniformities in the particle stream density and interferes with the use of an electron beam evaporation process.
A further process is know (K. Hiroshima: Controlled High-Tilt-Angle Nematic Alignment Compatible with Glass Frit Sealing, Jpn. J. Appl. Phys., Vol. 21, No. 12, 1982, pp. L761-L763), by which the substrate plate is rotated with an angle-dependent speed so that the azimuthal angle of the particle stream impinging on the plate surface continually changes.
Notwithstanding the fact that a completely uniform azimuthal orientation of the preferred direction over large plates with relatively short source-plate separations cannot be achieved in this manner, the necessary equipment to implement this process is complicated and costly since every plate must have its own rotatable holder and controlled drive. The number of plates that could be evaporated on in a single operation is therefore small.
Finally, a process for the selective coating of polarizer plates having parallel grooves is known from U.S. Pat. No. 3,046,839, in which the grooves are coated on one side with a metal layer by means of an obliquely impinging particle stream which forms the grid wires of the polarizer. In order to be able to coat the sides of the grooves as evenly as possible with respect to thickness and area, an aperture is provided between the source and the plate which limits the field of the particle stream. The plate is moved through this stream in a direction essentially parallel to the particle stream during the evaporation operation. The evaporated metal layer merely has the function of providing an opaque layer. To be sure, a uniform angle of incidence results from the displacement which is essentially parallel to the particle stream, but the azimuthal variation of the incident direction over the area of the plate remains unchanged. Applying this known process for the production of an orientation layer for liquid crystals would bring no improvement especially with regard to the important azimuthal orientation.